In gas turbine engines, for example, aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted rotary compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and drives the compressor. The hot exhaust gases flow from the back of the engine, providing thrust that propels the aircraft forward.
During operation of gas turbine engines, the metal parts of the engine, are in contact with hot, corrosive gases. The metal parts require particular attention to protect them from these combustion gases. These metallic parts include blades and vanes used to direct the flow of the hot gases, as well as other components such as shrouds and combustors.
In order to protect the metallic parts from the hot, oxidative and corrosive effects of the combustion gases, environmental coatings typically are applied to the metallic parts. These environmental coats may be produced by holding the part to be coated at temperature in an atmosphere rich in a certain element or elements, often aluminum. These elements diffuse into the surface of the part to form a diffusion coating, a process called chemical vapor deposition (CVD). In one form, the environmental coat is made of a diffusion nickel aluminide or platinum aluminide. Diffusing Al into the substrate has also proven effective against high temperature oxidation in addition to improving adherence of the ceramic TBC. The CVD bond coat surface forms an aluminum oxide scale during exposure to oxygen containing atmospheres at elevated temperatures, providing increased resistance to further high temperature oxidation. Other well-known methods are utilized to form diffusion aluminide coatings. While not meant to be inclusive, some other of these methods include “over the pack” aluminizing, pack aluminizing, flash electroplating of nickel and platinum onto a substrate followed by application of aluminum by one of these well-known methods. Frequently, these environmental layers also serve as a bond coat in a thermal barrier system that utilizes a thermal barrier coating over the diffusion aluminide layer, thereby impeding the transfer of heat from the hot exhaust gases to the parts by providing an insulating layer and allowing the exhaust gases to be hotter than would otherwise be possible.
Chipping of the protective coating sometimes occurs during the life of the part. This chipping damage may he caused during machining of the aluminide coated component, by poor handling of the component during subsequent manufacturing processes, during routine maintenance or through the normal operational environment of the turbine component. When repairing chipping damage, it is not cost effective to remove the remaining undamaged coating and re-coat the entire turbine component. Instead, localized repair of only the damaged surface is attempted. Current practice for localized repair of aluminide coating on damaged or selective areas of the turbine component is exemplified by, for example, U.S. Pat. Nos. 5,334,417 and 6,045,863, involving slurry or tape processes.
For example, in a proprietary commercial form presently used by the Assignee of the present invention, a self-adhesive halide activated or non-activated iron aluminum alloy containing about 55–57 wt. % aluminum tape or, alternatively, a cobalt aluminum alloy containing about 50–60 wt. % aluminum tape is placed on the selective area to be coated. The taped component is placed inside a metal coating box or can and packed in an inert aluminide oxide powder to hold the tape in place and mask the machined area during the coating operation. The coating box or can is heated to between about 1800° F. and about 2000° F. under an inert (reducing) atmosphere for a time sufficient to permit diffusion of aluminum to achieve the desired aluminide coating thickness, typically about three to eight hours just to accomplish the soak at the temperature to achieve a coating thickness of about 1 to about 3 mils. One cycle can take from 14–32 hours.
However, built up stress from the thermal expansion mismatch between the engine component and the inert aluminum oxide powder creates warpage or distortion of the selectively coated engine component, making the component unusable in the engine. The unusable component must be discarded at great cost.
What is needed are improved methods to apply diffusion aluminide coating to a selective area of an engine component, which results in little or no warpage or distortion of the component, hence, less waste. The present invention fulfills this need, and further provides related advantages.